1. Field of the Invention
The present invention relates to an apparatus for controlling energization of a heating element, such as a heater.
2. Related Background Art
Conventionally, a so-called electrophotographic image forming apparatus comprises: an image forming unit, for forming a toner image on a photosensitive drum and transferring the toner image to a recording medium (sheet member) to form an image on its surface; and a fixing unit, located below, for heating the toner image formed on the recording medium so as to fix the toner to its surface.
The temperature at the heating section of the fixing unit is controlled so that a constant heat quantity is transmitted to the recording medium. A temperature detector, such as a thermistor, controls the temperature of the heating section. However, a disconnection of the thermistor and an abnormal temperature rise at the heating section can not be identified as separate malfunctions, and are detected as the same abnormality because the thermistor has a negative temperature coefficient.
When an abnormality signal is generated at the thermistor, therefore, it is impossible to immediately determine whether it is for a disconnection or for an abnormal temperature rise. The abnormality can not be handled until the cause is found by checking the wiring and the temperature.
Furthermore, the conventional image forming apparatus employs, as a safety mechanism for preventing the temperature of a fixing roller from exceeding a set temperature, a high temperature detection thermistor in addition to a temperature control thermistor, or an energizing controller having a temperature switch connected in series with a heater to halt the energization of the heater when a temperature rises unexpectedly high.
However, in the energizing controller of the conventional image forming apparatus, when the temperature switch is activated as a safety mechanism in the event an operating failure occurs in a temperature control thermistor, the operating speed of the temperature switch is slower than is that of the thermistor, so that heat hysteresis continues to affect the fixing roller and an image forming problem occurs.
Also employed is a mechanism for detecting, as a temperature detection abnormality, the failure of the temperature of the temperature control thermistor to rise, even though a heater energizing signal is received, and for halting the energization of a heater. With this mechanism, however, the magnitude of the detected temperature error is increased by the entrance of a foreign substance between a temperature control thermistor and a fixing roller, so that heat hysteresis continues to affect the fixing roller and an image forming problem occurs.